Colloidal compositions for electroless deposition

ABSTRACT

Metallic surfaces are imparted to non-conductors or dielectric substrates by electroless(chemical) plating process comprised of coating the substrates with colloid(s) of non-precious metals and wherein the colloids are prepared in a manner as to impart resistance towards further deterioration.

REFERENCE TO PRIOR APPLICATIONS

This application is a continuation of Ser. No. 07/171,333, filed Mar.21, 1988, abandoned, which is a continuation of Ser. No. 06/948,089,filed Dec. 31, 1986, abandoned, which is a continuation of Ser. No.06/878,865, filed June 26, 1986, abandoned, which is acontinuation-in-part of Ser. No. 06/264,214, filed May 15, 1981abandoned, which is a divisional application of Ser. No. 06/137,447,filed Apr. 4, 1990, abandoned, which is a divisional application of Ser.No. 05/938,890, filed Aug. 3, 1978 and issued May 5, 1981 as U.S. Pat.No. 4,265,942.

BACKGROUND OF THE INVENTION

In the plating of dielectric substrates by chemical (electroless)plating it is well known that suitable catalytic pretreatment is aprerequisite for effective electroless metal deposition. Such practicesare well known and accepted in the art.

In examining the prior art for catalytic pretreatment it appears thatwhile different procedures have been used, the incorporation of preciousmetals (e.g., palladium containing solutions) was common to allprocedures. One catalytic system of particular interest is the two stepprocess as disclosed in U.S. Pat. No. 3,011,920. In the processdisclosed, a colloidal solution composed of tin (II) and precious metalsalts, generally with hydrochloric acid, is used. The effective catalystis proposed to be a colloid of an elemental precious metal (e.g.,palladium) stabilized by the excess stannous chloride present in themedium. While the system disclosed in U.S. Pat. No. 3,011,920 has beenquite popular in commercial practices, rising costs of precious metal,instabilities due to air oxidation, and miscellaneous productreliability problems have led to the quest for new systems in which theuse of precious metal as well as hydrochloric acid would be completelyeliminated.

In meeting this objective it was found, as described in U.S. Pat. Nos.3,958,048, 3,993,491, 3,993,799, and 4,087,586 that colloidal systemsbased upon non-precious metals could constitute the basis for newcommercial plating processes. More specifically, it was found andreported that colloidal compositions of non-precious metals (preferablyselected from the group of copper, cobalt, iron, nickel and manganese)may be used in the direct replacement of the tin/palladium colloidfollowed by a treatment (which may be optional) in a suitable reducing(activating medium). In the latter treatment a precursor derived fromthe colloidal dispersion constitutes the catalytic sites useful in theinitiation of plating. In the reducing medium, reduction of the ionicportion of the adduct derived from adsorption in the collodial mediumtakes place, or surface activation, which results in active nucleatingsites capable of initiation of the electroless process. Alternatively,the second step may encompass the selective dissolution of a colloidstabilizer(s) thereby exposing the catalytic nucleus of the colloid, orcontacting with a solution comprising soluble compound(s) of catalyticmetal. Hence, it should be recognized that the step of activation whichis optional generally is intended to reduce the induction time for theactual electroless metal build-up or to remove weakly adsorbed colloids,thereby preventing their contaminating the plating bath(s).

The colloidal nucleus may be in a form of a metallic (elemental) state,or compound bearing the metal, or alloy, and mixtures thereof and themetal(s) must be of a catalytic nature in at least one of its possibleoxidation states.

In reviewing the teaching disclosed in the aforementioned issued patentswhich are included herein by reference, it is recognized that many ofthe inherent disadvantages associated with the palladium based catalystsare eliminated. It is further recognized that based upon practices inthis art it is further essential that any catalytic system shouldmaintain its properties especially with storage (e.g., several months)and shipment under conditions of substantial temperature fluctuation. Itis thus highly desirable to have a medium in which good colloidalstability would be maintained, and which at the same time has sufficientcatalytic activity to be used in the plating process. I have observedthat as one increases stability, activity is decreased thereby making itdifficult to meet both requirements in a single preparation step.

For example, I have observed that with successful synthesis of activeplating colloids, there is generally a limited stability (for long termpurposes) due to coagulation which takes place leading to precipitation,with, of course, change in particle size and distribution during thecoagulation process. Also, at times dissolution of the colloidal statemay also take place with time. In addition, I have noted that highlystable colloidal dispersions have shown limited catalytic activity whenused in accordance with U.S. Pat. No. 3,993,799 with a moderateconcentration of reducing medium or activating medium or the omission ofany secondary step. Similar trends were also noted in U.S. Pat. No.3,948,048 on the interrelationship between reactivity and stability. Infact, in U.S. Pat. No. 3,958,048 most of the illustrated examples, whenrepeated, lost their colloidal character and became true solutionswithin 24 hours probably due to the interaction of the colloid with airor more particularly with oxygen. Many times the deterioration of thecolloid is manifested in visible color change(s). Hence, simple visualobservation could determine whether a colloid is deteriorating.

It is thus highly desirable to provide stable colloidal dispersionswhich would maintain their integrity and resist deterioration byprecipitation and/or contact with air. Such colloids may be useful inelectroless plating process, catalysis, or any other processes utilizingcolloids. It is further desirable to obtain dispersions with very fineparticle size distributions. Small size dispersions are particularlyuseful in adsorption processes and catalysis.

While not wishing to be repetitious, the following are included hereinby reference: U.S. Pat. Nos. 3,011,920, 3,993,799, 3,524,754, 3,958,048,3,993,491, 3,993,801, 4,087,586, 4,048,354, British Patent 1,078,439,and copending application Ser. No. 712,131 now U.S. Pat. No. 4,136,216,Ser. No. 820,904 now U.S. Pat. No. 4,131,699, Ser. Nos. 833,905 and854,909 now U.S. Pat. No. 4,132,832.

SUMMARY OF THE INVENTION

Novel colloidal compositions useful in electroless plating processes aredisclosed which are particularly stable against further deterioration.

DETAILED DESCRIPTION OF THE INVENTION

The process and composition of the present invention is applicable tothe metallic plating of dielectric substrates by autocatalytic or asmore commonly known, electroless (chemical) plating. Such processes arewell known in the art and they produce a wide variety of productsvarying from printed circuitry arrays to decorative plated plasticsparts.

In some of the above applications the colloids are used afterpreparation without any further change; however, in others changes maybe induced after preparation, e.g., change in the oxidation state of aportion of the colloid.

Although there are many methods for the preparation of colloidaldispersion, the use of the precipitation (chemical) method has beenquite popular. In the latter method, the insoluble phase is developed(nucleated) through the interaction of at least two reactants, e.g., ametal compound with a metal reducing agent, or alternately, a solublemetal salt with an alkaline agent. Both reactants should preferably besoluble in a suitable solvent. For general survey of preparatory methodssee B. Jirgensons and M. E. Straumanis, "A Short Textbook of ColloidChemistry," 2nd Edition, The MacMillan Company, New York (1962). Thepresent invention would be illustrated through the preparation ofcolloidal dispersions by precipitation (chemical) method; however, it isclear that the invention is not limited to the preparation methodselected.

The method described in this invention may be applied to any of severalcolloidal compositions of non-precious metals; the metal may be part ofa compound, alloy, or in the metallic state, as well as combinationsthereof. Preferred metals are those which are catalytic for electrolessmetal deposition. Such metals are well known in the art and they arerecited in U.S. Pat. Nos. 3,011,920, 3,993,799 and many others.

The term "colloid stabilizer" as used herein is intended to encompasssubstances which alter the characteristics of the colloid so as toprevent, delay, or minimize their coagulation and precipitation.Stabilizers may be organic or inorganic substances and mixtures thereof.It is believed that these stabilizers are adsorbed onto the surface ofthe colloid thereby altering the surface charge and hence theirstability. Stabilizers contemplated by the present process andcomposition may include but are not limited to secondary colloids,protein, gelatin, agar agar, gum arabic, surfactants, sugars andpolyalcohols (glycerol), and miscellaneous chemicals derived from wood,e.g., lignin, hemicellulose. It is noted that gelatin is a form ofprotein. In the general sense it is recognized that stabilizers areinherently adsorbed onto the nucleus of the colloid or participatewithin the double-layer structure of the colloid. Moreover, it isrecognized that in the colloidal dispersion preferably at least onecolloid stabilizer must be present.

The term "surfactant" (or surface active agent) as used herein generallyrefers to substances which are capable of lowering the surface tensionof a liquid or the interfacial tension between two liquids. All usefulsurfactants possess the common feature of a water-soluble (hydrophilic)group attached to an organic (hydrophobic) chain. Surfactants as usedherein are also intended to .encompass detergents, dispersants, andemulsifying agents regardless of whether or not they lower the surfacetension of a liquid (e.g., water).

The term "primary metal" or "primary metal ions" as used hereingenerally refers to metal selected from the group consisting of copper,nickel, cobalt and iron and mixtures thereof. These metals exist in thecolloidal catalytic composition as a reaction product which may be inthe elemental state, compound, or alloy, and mixtures thereof. For asource of the primary metals soluble or insoluble compounds bearing suchmetals may be used.

The term "secondary metal" or "secondary metal ions" as used hereingenerally refers to metals selected from the group of metals selectedfrom Groups IIIA, IVA, and VA of the Periodic Table of the Elements andpreferably selected from the group consisting of antimony, lead,thallium, tin and chromium, and mixtures thereof. These metals may existin the colloidal catalytic composition as a reaction product, along withany of the primary metals and further they may be in the elementalstate, compound, or alloy, and mixtures thereof. For a source of thesecondary metals soluble or insoluble compounds bearing said metals maybe used. It is also recognized that other metals such as zinc, cadmium,indium, and bismuth are similar in their chemical properties to theabove secondary metals. Hence, the substitution of these metals fallswithin the spirit of the present invention. In preparing the colloidalcomposition and the utilization of the secondary metal (s), it ispreferable to use these metals in compounds in which they are in thelowest oxidation state, e.g., Cr in +3 oxidation state rather than in +6oxidation state. While generally these secondary metal(s) are not asactive as the primary metal(s), their incorporation generally results inmaintaining the integrity of the colloids and in particular averting thedeterioration of the colloids. While I do not wish to be bound bytheory, it is believed that the secondary metal(s) react chemically withthe primary metal(s) leading to a reaction product(s) which provideswith the improved results against deterioration. The reaction product(s)may be an alloy(s) or compound(s) of these metals. It is also noted thatmany of the secondary metals, when used without reacting with theprimary metal(s), or added post colloid nucleation, act as inhibitors asdemonstrated in Ser. No. 833,905. These metal(s) may exist in a wideconcentration range relative to the primary metal(s).

The term "precipitating agent" as used herein refers to chemicalsubstances which upon their interaction with compounds bearing thecatalytic metal(s) (primary and/or secondary metals) lead to theformation of an insoluble (colloidal) phase in the solvent (e.g.,water). Any of several chemical substances may be used ranging fromreducing agents (e.g., hydrides and its derivatives, hypophosphorousacid and its derivatives, hydrazine, formaldehyde, tannic acid,dithionate, sulfites, etc.) to hydroxides, sulfides, chromates,phosphate and others. Also, active metals may be used as possiblereducing agent, e.g., zinc with copper ions. Depending upon the natureof the precipitating agent any of several types of insoluble phases maybe formed. The use of precipitating agents is well documented and isfurther demonstrated in the references included by reference in thepresent application. In the formation of the colloidal state, many timesthermal energy must be supplied as to overcome the activation energyrequired for nucleation of the insoluble phase.

Generally speaking it is noted that non-conductor substrates may beclassified as vitreous type and organic type, depending upon theresidual charge on the surface. The former generally constitutematerials which are ceramic, glass, and the like, which generally arenegative surfaces due to oxygen exposed at the interface. By contrast,the second group generally constitute organic materials which ingeneral, and especially upon etching of such substrates, are positivelycharged. Hence, for effective adsorption it should be recognized that itis generally preferable to use colloids which are charged in theopposite charge of the substrate surface which is to be metallized,though intermediate "layers" may be used onto the substrate which alterits effective charge.

The following examples are provided to illustrate the findings of thepresent invention. These examples are not to be taken as in limitationof the invention, but it should be recognized that the inventionencompasses various combinations thereof and the concentrations may bevaried.

EXAMPLE 1

An alumina ceramic substrate was immersed into a colloidal compositioncomprising the reaction product of the following components. Thereaction was carried forth at about 50° C.

    ______________________________________                                               CuCl.sub.2     0.04M                                                          Sn(BF.sub.4).sub.2                                                                           0.032M                                                         Gelatin        5.0 g/l                                                        NaBH.sub.4     0.05M                                                          NaOH           0.5M                                                    ______________________________________                                    

The substrate was immersed for several minutes and thereafter waterrinsed, and directly immersed into a commercial electroless copper(Enplate 404) operating at room temperature. Spontaneous plating ofcopper was noted.

In the course of evaluating compositions of tin/copper it wassurprisingly and unexpectedly found that:

1. The product exhibits a greater stability when exposed to air incomparison to copper alone.

2. Similar improvement of the copper colloid was achieved by thecombination of two distinct colloidal compositions: the first was copperalone (reduced with NaHB₄) and the second was of tin alone(reduced withNaBH₄). Hence, both modes of preparation fall into the spirit of thepresent invention.

3. Examination of tin/copper colloidal product by electron microscopydiffraction and transmission modes appears to yield new interactionproduct(s), probably an alloy of these metals. Specifically, it wasnoted that the diffraction patterns showed the disappearance of certaind-spacings as well as the formation of new lines. Also noted wereparticles of about 50Å in size whereas the particles of the tin colloidalone were much greater in size.

4. The concentration for tin relative to copper can be used over a widerange.

Substitution of nickel or iron or cobalt for copper or in addition tocOpper is self evident.

It should be recognized that regardless of the mode of preparation boththe copper(primary metal) and the tin(secondary metal) are reduced to anelemental or a boride type compound(s).

EXAMPLE 2

A colloidal composition was prepared comprising the reaction product ofthe admixture comprising

    ______________________________________                                        CuCl.sub.2            0.04M                                                   TlNO.sub.3            0.002M                                                  Arabic Gum            11.8 g/l                                                NaBH.sub.4            0.04M                                                   NaOH                  0.5M                                                    ______________________________________                                    

The resulting pH was 12 and the nucleation reaction was carried outabove room temperature. Comparison of this resulting colloidal productto copper alone showed a greater stability towards exposure to air.

EXAMPLE 3

A colloidal composition comprising of copper and tin was prepared by theadmixing of:

    ______________________________________                                        CuCl.sub.2 · 2H.sub.2 O                                                                    10.6 g/l                                                Sodium citrate          25 g/l                                                SnCl.sub.2 · 2H.sub.2 O                                                                    19.2 g/l                                                ______________________________________                                    

Prior to the addition of the tin, the solution (at pH 14) was heated toabout 100° C. and final pH adjustment of the product was made to about12.5. The formation of a brown-gray colloid was noted.

Immersion of an etched ABS substrate followed by electroless coppershowed that plating has taken place. The preparation of the abovecolloid was made based upon Weiser, Vol. I, "Inorganic ColloidChemistry", p.137, John Wiley & Sons (1933).

It should be recognized that in this example the reaction product is notthe same as other examples in this application e.g., Example 1.Specifically, in this example, the copper ions are reduced to yield thecolloidal phase at the expense of the tin ions whereas the latter areoxidized to the stannic state.

EXAMPLE 4

A colloidal composition comprising the admixture of the followingchemicals was nucleated above room temperature. The nickel was addedpost copper colloid nucleation and the final pH was adjusted to 8.0.

    ______________________________________                                               CuCl.sub.2    0.04M                                                           NiCl.sub.2    0.01M                                                           NaBH.sub.4    0.039M                                                          NaOH          0.196M                                                          (NH.sub.2).sub.2 CS                                                                         0.0067M                                                         Orzan-S       12.0 g/l                                                 ______________________________________                                    

This product showed a superior stability to deterioration in comparisonto the same product in the absence of added (NH₂)₂ CS. While I do notwish to be bound by theory, it is believed that the addition of thiourearesults in adsorption onto the colloid and provides protection againstfurther deterioration. Homologs of thiourea were also used, e.g., (NH₂)₂CO and (NH₂)₂ CSe. However, their effectiveness was not as good as thethiourea. It is further realized that it is the thio group ##STR1## thatprovides the present effect and thus substituting other thio containingcompounds falls within the spirit of this invention. R₁ and R₂ may bevarious groups from alkyl, amines, aromatics, hydrogen, halogen, andmixtures thereof.

EXAMPLE 5

A colloidal composition comprising the reaction product resulting fromthe admixture of the following reactants was prepared. Nucleation tookplace above room temperature. Final pH was about 12. The resultingcolloidal composition showed a greater visual stability in comparison tothe same without the added antimony. Hence, antimony in combination withthe copper primary metal is a useful combination.

    ______________________________________                                        CuCl.sub.2            0.04M                                                   SbCl.sub.3            26.8 g/l                                                Arabic Gum            8.8 g/l                                                 NaBH.sub.4            0.049M                                                  NaOH                  0.49M                                                   ______________________________________                                    

EXAMPLE 6

A colloidal composition similar to above was prepared except that leadwas used instead of the antimony trichloride. Specifically, the lead wasPb (from fluoroborate solution) at 0.04M. Final pH about 12 and greaterstability was noted. Substitution of equimolar Cr⁺³ for the lead alsoshowed a tendency for the formation of a more stable product(s) relativeto copper alone.

While the above examples are provided to illustrate the novel aspects ofthe present invention one skilled in the art should recognize that manyother chemicals and means may be used to achieve the useful results ofthis invention. Such adaptations and modifications fall within thespirit of the present invention. Furthermore, it is possible that thecolloidal composition may be prepared by the suspension of an availabledry powder or semi-dried powder in a suitable solvent; such approach incombination with the composition disclosed falls within the spirit ofthis invention. In using the present invention both soluble andinsoluble compounds may be used as starting chemicals for the colloidpreparation though soluble compounds are preferred. Moreover, any ofseveral oxidation states may be applied (e.g., Cu(I) and Cu(II)).

It is further noted as recognized in U.S. Pat. Nos. 3,993,491 and3,993,799 that compatible electroless plating compositions should beused for achieving best results.

It is further recognized that in the event that the insolublephase(colloid) is formed with the assistance of a precipitating agent,the solvent to carry forth the reaction may be of aqueous or non-aqueoustype. Moreover, contemplating the present invention the colloidalcomposition may be dispersed in aqueous or non-aqueous solvents.

What I claim is:
 1. A colloidal composition useful in the metallizationof an electrically non-conductive substrate wherein said colloidalcomposition is the reaction product resulting from the admixture in aliquid medium comprising at least one primary metal and at least onesecondary metal, said metals being in positive oxidation states, andwherein said primary metal is selected from the group consisting ofcopper, nickel, iron, and cobalt and mixtures thereof and wherein saidsecondary metal is selected from the group consisting of aluminum,gallium, indium, germanium, lead, antimony, bismuth, chromium, tin,thallium, and mixtures thereof, and further wherein said primary metaland said secondary metal in said resultant colloidal composition are ina reduced oxidation state derived through the chemical reduction in theadmixture of both the primary and secondary metals both metals havingbeen initially introduced in a higher oxidation state than theiroxidation states in the resultant colloidal composition.
 2. A method forimproving the stability of a colloidal dispersion comprising a primarymetal selected from the group consisting of copper, nickel, cobalt, andiron including the step comprising of the admixing of a secondary metalselected from the group consisting of aluminum, gallium, indium,germanium, lead, antimony, bismuth, chromium, tin, thallium, andmixtures thereof, and further wherein said primary metal and saidsecondary metal in said resultant colloidal composition are in a reducedoxidation state derived through the chemical reduction in the admixtureof both the primary and secondary metals, both metals having beeninitially introduced in a higher oxidation state than their oxidationstates in the resultant colloidal dispersion.